Norwalk virus has recently been assigned as the prototype species for a new genus in the Caliciviridae, provisionally named ?Norwalk-like viruses.? The ?Norwalk-like? human caliciviruses are the major cause of nonbacterial epidemic gastroenteritis that occurs in family, school, institutional, or community-wide outbreaks affecting adults, school-aged children, and some young children, as well. Epidemiologic studies of these viruses have been hampered by the inability to grow the human caliciviruses in cell culture. A molecular approach using recombinant proteins for the development of diagnostic assays has enabled large-scale epidemiologic studies that are defining the extent of antigenic diversity among the human caliciviruses and the role of this diversity in the natural history of these viruses. The initial work of our group focused on the genetic characterization of human calicivirus strains such as the Hawaii virus (HV), Toronto virus (TV), and Desert Shield virus (DSV) that were derived from gastroenteritis episodes that occurred in a family, hospital, and military setting, respectively. The ORF2 of the positive-sense RNA genome encoding the single capsid protein from each of these viruses was cloned and sequenced. The amino acid sequences of the capsid proteins of HV, TV, and DSV shared 48%, 47%, and 68% identity, respectively, with NV. In a provisional classification system based on sequence comparisons, NV and DSV belonged to ?Genogroup I,? and HV and TV belonged to ?Genogroup II.? We went on to clone the ORF2 of these viruses into a baculovirus transfer vector in order to develop recombinant virus-like particles (VLPs) for each strain. These VLPs were used in the development of diagnostic reagents for use in epidemiologic and laboratory studies. Several of our recent epidemiologic studies have been in collaboration with researchers in other laboratories such as the Centers for Disease Control, U.S., and the Public Health Service in London, England. The ORF2 Snow Mountain virus (SMV), representing a distinct serotype of human caliciviruses by IEM, was cloned into a baculovirus transfer vector and the SMV capsid protein was expressed. Antibodies were raised in guinea pigs against the purified SMV capsid protein. Studies are in progress to enable the production of virus-like particles (VLPs) that can be used in the development of enzyme-linked immunosorbent assays (ELISAs) to detect serologic evidence of infection with this virus. One immediate goal of our work is to complete the full-length genomic sequence analyses of our other reference strains of the human caliciviruses. Progress has been made in determining additional sequences of the Hawaii virus ORF1. We plan to generate a full-length clone of the Hawaii virus as a representative of the "Genogroup II" human caliciviruses. Thus far, there are no data regarding whether this group of viruses that are genetically distinct from NV might be recovered from an infectious clone. Progress has been made this year in the development of a taxonomy system for the family Caliciviridae. The principal investigator of this project led a team of international scientists as chair of the Caliciviridae Study Group for the International Committee on the Taxonomy of Viruses (ICTV). The family Caliciviridae has been divided into four new genera, represented by Norwalk virus, Sapporo virus, vesicular exanthema of swine virus, and rabbit hemorrhagic disease virus. This system, based on phylogenetic relationships and genomic characteristics, should lead to a better understanding of the relatedness of the human caliciviruses with other caliciviruses in nature and may give insight into the origin and evolution of these viruses as molecular epidemiologic studies progress.